Facile Exfoliation and Noncovalent Superacid Functionalization of Boron Nitride Nanosheets and Their Use for Highly Thermally Conductive and Electrically Insulating Polymer Nanocomposites

Highly Thermally Conductive Yet Electrically Insulating Polymer/Boron Nitride Nanosheets Nanocomposite Films for Improved Thermal Management Capability

Thermally conductive yet electrically insulating polymer composites are urgently required for thermal management applications of modern electrical systems and electronic devices because of their multifunctionality and ease of processing. However, the thermal conductivity enhancement of polymer composites is usually at the price of the loss of lightweight, the deterioration of flexibility, and electrical insulation. Here we report advanced polymer nanocomposites containing orientated boron nitride nanosheets (BNNSs), which simultaneously exhibit high thermal conductivity enhancement, excellent electrical insulation, and outstanding flexibility. These nanocomposite films can be easily constructed by electrospinning polymer/BNNSs nanocomposite fibers, vertically folding the electrospun nanocomposite fibers and the subsequent pressing. The nanocomposite films exhibit thickness-dependent in-plane thermal conductivity, which can reach 16.3 W/(m·K) in the 18 μm thick nanocomposite film with 33 wt % BNNSs. In addition, the nanocomposite films have superior electrically insulating properties compared with the pristine polymer, such as reduced dielectric loss, increased electrical resistivity, and enhanced breakdown strength. The strong thermal management capability of the nanocomposite film was demonstrated in switching power supply, which showed the importance of high in-plane thermal conductivity in thermal management of high-power density electronic devices.

Construction of multifunctional boron nitride nanosheet towards reducing toxic volatiles (CO and HCN) generation and fire hazard of thermoplastic polyurethane

Considerable toxic volatiles (CO and HCN) generation and high fire hazard has definitely compromised the application of thermoplastic polyurethane (TPU). Here, a novel functionalization strategy for bulky h-BN is adopted to obtain the multifunctional CPBN, aiming at the flame retardancy reinforcement of TPU. The multifunctional CPBN is successfully prepared via the wrapping of phytic acid doped polypyrrole shell, following with the adsorption of copper ions. The obviously decreased peak heat release rate, peak smoke production rate and total smoke production values, obtained from cone test, confirms the reduced fire hazard of TPU composite with CPBN. The dramatic suppressions on CO and HCN releases can also be observed from TG-IR test. Tensile test demonstrates that adding CPBN favors the reinforcement in mechanical property of TPU. Thus, the concurrent improvements in flame retardancy and mechanical performance are achieved by incorporating CPBN. This work opens up new avenues for the functionalization of h-BN, and thus facilitates its promising applications in polymer-matrix composite.

Hexagonal Boron Nitride/Microfibril Cellulose/Poly(vinyl alcohol) Ternary Composite Film with Thermal Conductivity and Flexibility

Microfibril cellulose (MFC), which is detrimental to soil cultivation and environmental protection, is derived from waste pineapple leaves. Hexagonal boron nitride (h-BN) was modified with polydopamine (PDA)-PDA@h-BN named pBN, and then combined with MFC to prepare a novel hybrid powder. The effect of PDA on h-BN and the binding effect between pBN and MFC were characterized by X-ray photoelectron spectroscopy (XPS), Thermogravimetric (TG), scanning electron microscopy (SEM), and Fourier Transform-Infrared (FT-IR). Poly (vinyl alcohol) (PVA) was used as an eco-friendly polymeric matrix to prepare a pBN-MFC-PVA composite film. The mechanical strength, hydrophobicity, and thermal conductivity of the film were studied and the results confirmed that h-BN was chemically modified with PDA and was uniformly distributed along the MFC. The thermal conductivity of the pBN-MFC-PVA composite film increased with the addition of a pBN-MFC novel powder. MFC acted as "guides" to mitigate the h-BN agglomerate. In addition to the possible usage in the pBN-MFC-PVA composite film itself, the pBN-MFC hybrid powder may be a potential filler candidate for manufacturing thermal interface materials and wearable devices or protective materials.

Introduction of Protonated Sites on Exfoliated, Large-Area Sheets of Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) sheets possess an exclusive set of properties, including wide energy band gap, high optical transparency, high dielectric breakdown strength, high thermal conductivity, UV cathodoluminescence, and pronounced thermochemical stability. However, functionalization of large h-BN layers has remained a challenge due to their chemical resistance and unavailable molecular-binding sites. Here we report on the protonation of h-BN via treatment with chlorosulfonic acid that not only exfoliates "large" h-BNs (up to 10 000 μm²) at high yields (∼23%) but also results in their covalent functionalization by introducing four forms of aminated nitrogen (N) sites within the h-BN lattice: sp²-delocalized and sp³-quaternary protonation on internal N sites (>N⁺═ and >NH⁺-) and pyridinic-like protonation on the edge N sites (═NH⁺- and -NH-). The presence of these groups transforms the chemically passive h-BN sheets to their chemically active form, which as demonstrated here can be used as scaffolds for forming composites with plasmonic gold nanoparticles and organic dye molecules. The dispersion of h-BNs exhibits an optical energy band gap of 5.74 eV and a zeta potential of ζ = +36.25 mV at pH = 6.1 (ζ_max = +150 mV), confirming high dispersion stability. We envision that these two-dimensional nanomaterials with an atomically packed honeycomb lattice and high-energy band gap will evolve next-generation applications in controlled-UV emission, atomic-tunneling-barrier devices, ultrathin controlled-permeability membranes, and thermochemically resistive transparent coatings.

Fluorinated Carbon Nanotube/Nanofibrillated Cellulose Composite Film with Enhanced Toughness, Superior Thermal Conductivity, and Electrical Insulation

Recently, graphene and carbon nanotubes (CNTs) promise considerable application potentials in the highly efficient thermal management of high-power devices because of their superb thermal conductivity (TC). However, the high electrical conductivity hampers their use in some fields where electrical insulating components are always required. Herein, to coordinate the thermal and electrical conductivity of CNT, fluorinated CNT (FCNT) was first used as a thermally conductive filler to prepare composite film with nanofibrillated celluloses (NFCs) via facile vacuum-assisted filtration. The obtained composite film shows a well-organized layered structure of the building blocks along the planar direction. Moreover, the one-dimensional structure of NFCs and the strong interaction of NFCs and FCNTs ensure sufficient connection between FCNT themselves and the reduced interfacial thermal resistance of NFCs/FCNTs, so that efficient heat transfer pathways can be well reserved, leading to simultaneous accessibility of high in-plane TC of 14.1 W m^-1 K^-1 and favorable electrical insulation property at an FCNT content of 35 wt %. Despite such a high FCNT loading, the strong interaction between NFCs and FCNTs enables the composite film to possess enhanced toughness, reliable mechanical strength, and flexibility. Therefore, we think that these outstanding comprehensive properties guarantee that the prepared composite film has promising applications in heat dissipation of next-generation portable and collapsible electronic devices.

Particle Packing Theory Guided Thermal Conductive Polymer Preparation and Related Properties

Thermal conductive polymer composites are satisfying for thermal management of electronic devices. However, how to choose the sizes of thermal conductive fillers to get a high thermal conductivity of composites are still clueless and poor filler size matching will also affect the processability of the composites. Closest packing model was used to guide multiscale thermal conductive particles filling silicone rubber in this work. A highest thermal conductivity of 1.381 W·m^-1·K^-1 at filler loading of 50 vol % was determined among nine comparing formulations. The fillers with small particle size filled the interspaces of fillers with large particle size to form more complete thermal conduction paths and heat dissipation was increased. The apparent densities and rheological tests further verified the effectiveness of closest packing model. This study provides theoretical guidance for thermal conductive polymer composites to achieve high thermal conductivity and good processability, which has an important practical application.

Highly Thermally Conductive Composite Films Based on Nanofibrillated Cellulose in Situ Coated with a Small Amount of Silver Nanoparticles

In this paper, a freestanding flexible nanofibrillated cellulose (NFC)/silver (Ag) composite film with high thermal conductivity (TC) was prepared using the NFC that was in situ coated with a small amount of Ag nanoparticles through mussel-inspired chemistry of dopamine. The results demonstrated that Ag nanoparticles were homogeneously coated on the surface of NFC nanofibers and their incorporation had little influence on the film-forming ability of NFC. The NFC decorated with Ag nanoparticles could easily form thermally conductive pathways in the composite films, and the resultant films containing only 2.0 vol % of Ag showed a high in-plane TC value of 6.0 W/(m·K), which was 4 times that of pure NFC film. Moreover, the composite films exhibited relatively high strength and flexibility. The highly thermally conductive NFC/Ag composite films possess potential applications as lateral heat spreaders in flexible electronic equipment.

Dynamic tungsten diselenide nanomaterials: supramolecular assembly-induced structural transition over exfoliated two-dimensional nanosheets

Supramolecular polymers can easily control the lamellar microstructures on exfoliated tungsten diselenide nanosheets.

A simple and effective method for direct exfoliation of tungsten diselenide (WSe₂) into few-layered nanosheets has been successfully developed by employing a low molecular weight adenine-functionalized supramolecular polymer (A-PPG). In this study, we discover A-PPG can self-assemble into a long-range, ordered lamellar microstructure on the surface of WSe₂ due to the efficient non-covalent interactions between A-PPG and WSe₂. Morphological and light scattering studies confirmed the dynamic self-assembly behavior of A-PPG has the capacity to efficiently manipulate the transition between contractile and extended lamellar microstructures on the surface of metallic 1T-phase and semiconducting 2H-phase WSe₂ nanosheets, respectively. The extent of WSe₂ exfoliation can be easily controlled by systematically adjusting the amount of A-PPG in the composites, to obtain nanocomposites with the desired functional characteristics. In addition, the resulting composites possess unique liquid–solid phase transition behavior and excellent thermoreversible properties, revealing the self-assembled lamellar structure of A-PPG functions as a critical factor to manipulate and tailor the physical properties of exfoliated WSe₂. This newly developed method of producing exfoliated WSe₂ provides a useful conceptual and potential framework for developing WSe₂-based multifunctional nanocomposites to extend their application in solution-processed semiconductor devices.

Polymer composites based on hexagonal boron nitride and their application in thermally conductive composites

Hexagonal boron nitride (h-BN) is also referred to as "white graphite". Owing to its two-dimensional planar structure, its thermal conductivity along and perpendicular to a basal plane is anisotropic. However, h-BN exhibits properties that are distinct from those of graphite, such as electric insulation, superior antioxidative ability, and purely white appearance. These qualities render h-BN superior as a filler in composites that require thermal conductivity while exhibiting electric insulation. Since the thermal performance of composites is mainly affected by thermal pathways, this article begins with an overall introduction of the preparation of boron nitride nanosheets, followed by a review of the fabrication of h-BN-filled composites. Lastly, the construction of thermally conductive networks is discussed.

Hydrogen Bond-Regulated Boron Nitride Network Structures for Improved Thermal Conductive Property of Polyamide-imide Composites

Highly thermal conductive polymer composites with minimized content of fillers are desirable for handling the issue in thermal management in modern electronics. However, the difficulty of filler dispersion restricts the heat dissipation performance of thermoplastic composites and the intermolecular interaction is another crucial factor in this problem. In the present study, the hydrogen bond was used to regulate the formation of the three-dimensional boron nitride (3D BN) interconnected network to act as a high thermal conductive network in thermoplastic polyamide-imide (PAI) materials. The prepared electrical insulated PAI/3D-BN composites have a thermal conductivity (TC) of 1.17 W·m^-1·K^-1 at a low BN loading of 4 wt %/2 vol % and exhibit a thermal conductivity enhancement of 409%. We attribute the increased TC to the construction of 3D BN interconnected network and the hydrogen bond regulated between hydroxylated BN and polyvinyl alcohol, in which an effective thermal conductive network is constructed. This study provides a guided hydrogen bond strategy for thermally conductive polymer composites with good mechanical and electrical insulation properties in thermal management and other applications.

Synthesis and fluorescence properties of some difluoroboron β-diketonate complexes and composite containing PMMA

A series of difluoroboron β-diketonate complexes, containing the indon-β-diketonate ligand carrying methyl or methoxyl substituents was synthesized. The crystal structures of the complexes were confirmed by single crystal X-ray diffraction studies. The fluorescence properties of compounds were studied in solution state, solid state and on PMMA polymer matrix. The photophysical data of compounds 2a-2d exhibited strong fluorescence and photostability under the ultraviolet light (Hg lamp). The complex 2b showed higher fluorescence intensity in solution state as compared to other complexes of the series. The complexes 2c and 2d showed higher fluorescence intensity in the solid state, which are ascribed to the stronger π-π interactions between ligands in the solid state. The introduction of methoxyl or methyl groups on the benzene rings enhanced the absorption intensity, emission intensity, quantum yields and fluorescence lifetimes due to their electron-donating nature. Furthermore, the complex 2b was doped into the PMMA to produce hybrid materials, where the PMMA matrix acted as sensitizer for the central boron ion to enhance the fluorescence emission intensity and quantum yields.

Extraction of Boron Nitride Nanotubes and Fabrication of Macroscopic Articles Using Chlorosulfonic Acid

Due to recent advances in high-throughput synthesis, research on boron nitride nanotubes (BNNTs) is moving toward applications. One future goal is the assembly of macroscopic articles of high-aspect-ratio, pristine BNNTs. However, these articles are presently unattainable because of insufficient purification and fabrication methods. We introduce a solution process for extracting BNNTs from synthesis impurities without sonication or the use of surfactants and proceed to convert the extracted BNNTs into thin films. The solution process can also be used to convert as-synthesized material-which contains significant amounts of hexagonal boron nitride ( h-BN)-into mats and aerogels with controllable structure and dimension. The solution extraction method, combined with further advances in synthesis and purification, contributes to the development of all-BNNT macroscopic articles, such as fibers and 3-D structures.

Aqueous Phase Exfoliation of Two-Dimensional Materials Assisted by Thermoresponsive Polymeric Ionic Liquid and Their Applications in Stimuli-Responsive Hydrogels and Highly Thermally Conductive Films

With the increasing attention for various two-dimensional (2D) materials in recent years, developing a universal, facile, and eco-friendly method to exfoliate them into single- and few-layered nanosheets is becoming more and more urgent. Herein, we use a thermoresponsive polymeric ionic liquid (TRPIL) as a universal polymer surfactant to assist the high-efficiency exfoliation of molybdenum disulfide (MoS₂), graphite, and hexagonal boron nitride in an aqueous medium through consecutive sonication. In this case, the reliable interaction between 2D materials and the TRPIL would facilitate the exfoliation and simultaneously achieve a noncovalent functionalization of the exfoliated nanosheets. Interestingly, the dispersion stability of exfoliated nanosheet suspensions can be reversibly tuned by temperature because of the thermoresponsive phase transition behavior of the TRPIL. As a proof of potential applications, a temperature and photo-dual-responsive TRPIL/MoS₂ coloring hydrogel with robust mechanical property and an artificial nacre-like BN nanosheet film with high thermal conductivity were fabricated.

Hot pressing-induced alignment of hexagonal boron nitride in SEBS elastomer for superior thermally conductive composites

Orientational hBN/SEBS composite films embued with superior thermal conductivity and improved dimensional stability were prepared by hot-pressing treatment.

Shear-Assisted Production of Few-Layer Boron Nitride Nanosheets by Supercritical CO2 Exfoliation and Its Use for Thermally Conductive Epoxy Composites

Boron nitride nanosheets (BNNS) hold the similar two-dimensional structure as graphene and unique properties complementary to graphene, which makes it attractive in application ranging from electronics to energy storage. The exfoliation of boron nitride (BN) still remains challenge and hinders the applications of BNNS. In this work, the preparation of BNNS has been realized by a shear-assisted supercritical CO₂ exfoliation process, during which supercritical CO₂ intercalates and diffuses between boron nitride layers, and then the exfoliation of BN layers is obtained in the rapid depressurization process by overcoming the van der Waals forces. Our results indicate that the bulk boron nitride has been successfully exfoliated into thin nanosheets with an average 6 layers. It is found that the produced BNNS is well-dispersed in isopropyl alcohol (IPA) with a higher extinction coefficient compared with the bulk BN. Moreover, the BNNS/epoxy composite used as thermal interface materials has been prepared. The introduction of BNNS results in a 313% enhancement in thermal conductivity. Our results demonstrate that BNNS produced by supercritical CO₂ exfoliation show great potential applications for heat dissipation of high efficiency electronics.

BN Nanosheet/Polymer Films with Highly Anisotropic Thermal Conductivity for Thermal Management Applications

The development of advanced thermal transport materials is a global challenge. Two-dimensional nanomaterials have been demonstrated as promising candidates for thermal management applications. Here, we report a boron nitride (BN) nanosheet/polymer composite film with excellent flexibility and toughness prepared by vacuum-assisted filtration. The mechanical performance of the composite film is highly flexible and robust. It is noteworthy that the film exhibits highly anisotropic properties, with superior in-plane thermal conductivity of around 200 W m^-1 K^-1 and extremely low through-plane thermal conductivity of 1.0 W m^-1 K^-1, making this material an excellent candidate for thermal management in electronics. Importantly, the composite film shows fire-resistant properties. The newly developed unconventional flexible, tough, and refractory BN films are also promising for heat dissipation in a variety of applications.

Ultrahigh performance heat spreader based on gas-liquid exfoliation boron nitride nanosheets

Due to their high thermal conductivity and insulation performance, boron nitride nanosheets (BNNS) have great promise to fabricate thermal management equipment for application in power electronics. The liquid-phase exfoliation route has been regarded as the most commonly used approach to produce single and few-layered BNNS for many research fields. However, this process takes a long time, and the production yield is extremely low. In this work, an efficient technique to obtain few-layered (mostly < 5 layers), high-yield (∼33%), and plane-defect-free BNNS by the combination of liquid N₂ (L-N₂) gasification and liquid exfoliation was developed. The as-obtained BNNS suspensions could be vacuum filtered to make a thermal conductive film named a BNNS heat spreader which possessed a superior thermal conductivity of 61.2 W m^-1 K^-1 at room temperature. In addition, we also proved that the thermal conductivity of the BNNS heat spreader increased with the increase of density, creating an approach for fine tuning the thermal property of this heat spreader.

Vertically Aligned and Interconnected Boron Nitride Nanosheets for Advanced Flexible Nanocomposite Thermal Interface Materials

The continuous evolution toward semiconductor technology in the "more-than-Moore" era and rapidly increasing power density of modern electronic devices call for advanced thermal interface materials (TIMs). Here, we report a novel strategy to construct flexible polymer nanocomposite TIMs for advanced thermal management applications. First, aligned polyvinyl alcohol (PVA) supported and interconnected 2D boron nitride nanosheets (BNNSs) composite fiber membranes were fabricated by electrospinning. Then, the nanocomposite TIMs were constructed by rolling the PVA/BNNS composite fiber membranes to form cylinders and subsequently vacuum-assisted impregnation of polydimethylsiloxane (PDMS) into the porous cylinders. The nanocomposite TIMs not only exhibit a superhigh through-plane thermal conductivity enhancement of about 10 times at a low BNNS loading of 15.6 vol % in comparison with the pristine PDMS but also show excellent electrical insulating property (i.e., high volume electrical resistivity). The outstanding thermal management capability of the nanocomposite TIMs was practically confirmed by capturing the surface temperature variations of a working LED chip integrated with the nanocomposite TIMs.

Achieving a Collapsible, Strong, and Highly Thermally Conductive Film Based on Oriented Functionalized Boron Nitride Nanosheets and Cellulose Nanofiber

Boron nitride nanosheet (BNNS) films receive wide attention in both academia and industry because of their high thermal conductivity (TC) and good electrical insulation capability. However, the brittleness and low strength of the BNNS film largely limit its application. Herein, functionalized BNNSs (f-BNNSs) with a well-maintained in-plane crystalline structure were first prepared utilizing urea in the aqueous solution via ball-milling for the purpose of improving their stability in water and enhancing the interaction with the polymer matrix. Then, a biodegradable and highly thermally conductive film with an orderly oriented structure based on cellulose nanofibers (CNFs) and f-BNNSs was prepared just by simple vacuum-assisted filtration. The modification of the BNNS and the introduction of the CNF result in a better orientation of the f-BNNS, sufficient connection between f-BNNS themselves, and strong interaction between f-BNNS and CNF, which not only make the prepared composite film strong and tough but also possess higher in-plane TC. An increase of 70% in-plane TC, 63.2% tensile strength, and 77.8% elongation could be achieved for CNF/f-BNNS films, compared with that for CNF/BNNS films at the filler content of 70%. Although at such a high f-BNNS content, this composite film can be bended and folded. It is even more interesting to find that the in-plane TC could be greatly enhanced with the decrease of the thickness of the film, and a value of 30.25 W/m K can be achieved at the thickness of ∼30 μm for the film containing 70 wt % f-BNNS. We believe that this highly thermally conductive film with good strength and toughness could have potential applications in next-generation highly powerful and collapsible electronic devices.

Preparation of Highly Thermally Conductive Polymer Composite at Low Filler Content via a Self-Assembly Process between Polystyrene Microspheres and Boron Nitride Nanosheets

Rational distribution and orientation of boron nitride nanosheets (BNNSs) are very significant for a polymer/BNNS composite to obtain a high thermal conductivity at low filler content. In this paper, a high-performance thermal interface material based on exfoliated BNNSs and polystyrene (PS) microspheres was fabricated by latex blending and subsequent compression molding. In this case, BNNSs and PS microspheres first self-assembled to form the complex microspheres via strong electrostatic interactions between them. The as-prepared complex microspheres were further hot-pressed around the glass transition temperature, which brought the selective distribution of BNNSs at the interface of the deformed PS microspheres. As a consequence, a polymer composite with homogeneous dispersion and high in-plane orientation of BNNSs in PS matrix was obtained. Benefitted from this unique structure, the resultant composite exhibits a significant thermal conductivity enhancement of 8.0 W m^-1 K^-1 at a low filler content of 13.4 vol %. This facile method provides a new strategy to design and fabricate highly thermally conductive composites.

A Green Plastic Constructed from Cellulose and Functionalized Graphene with High Thermal Conductivity

It is urgent to fabricate a class of green plastics to substitute synthetic plastics with increasing awareness of sustainable development of an ecological environment and economy. In this work, a novel green plastic constructed from cellulose and functionalized graphene has been explored. The mechanical properties and thermal stability of the resultant cellulose/functionalized graphene composite plastics (CGPs) equal or even exceed those of synthetic plastics. Moreover, the in-plane thermal conductivity of CGPs can reach 9.0 W·m^-1·K^-1 with only 6 wt % functionalized graphene loading. These superior properties are attributed to the strong hydrogen-bonding interaction between cellulose and functionalized graphene, the uniform dispersion of functionalized graphene, and the alignment structure of CGPs. Given the promising synergistic performances and ecofriendly features of CGPs, we envisage that CGPs as novel green plastics could play important roles in thermal management devices.

Highly thermally conductive and electrically insulating polymer nanocomposites with boron nitride nanosheet/ionic liquid complexes

Boron nitride nanosheet (BNNS)/ionic liquid (IL)/polymer composites show significant enhancement of through-plane and in-plane thermal conductivities and electrical insulation.

Screen-Printable Electronic Ink of Ultrathin Boron Nitride Nanosheets

Two-dimensional materials play a vital role in the current electronic industry in the fabrication of devices. In the present work, we have exfoliated and stabilized the insulating hexagonal boron nitride (hBN) by means of a polymer-assisted liquid-phase technique. Further, the highly viscous ink of hBN was prepared, and its printability on various commercially available substrates was studied. The morphology of the printed patterns reveals the layered arrangement of hBN. The various electrical and dielectric characterizations, carried out on a metal-insulator-metal capacitor, testified its potential applications in various fields of printed electronics.